We conducted a longitudinal study to examine changes in the respiratory exchange ratio (RER) during progressively increasing body exertion in children and adolescents of female sex. In this analysis we only included 23 examinees for which we had all yearly measurements from examinee's age 9 years until 18 years of age. The data were analyzed according to the chronological and biological age. According to both criteria, the highest RER values were recorded at moments of maximum exertion and they did not increase with age. We found the highest RER values were in the year of the menarche. We interpret these results as related to the effect of estrogen. The beginning of sexual development involves a gradual increase in estrogen plasma concentrations. At one point serum levels of estrogen reach a level high, enough to allow for maximum RER values, i.e. causing the optimum anaerobic capacity of the examinee. This threshold estrogen value varies between individuals.